1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery provided with a porous heat resistance layer between a separator and an electrode.
2. Description of Related Art
Nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries and nickel hydrogen batteries are recently used as so-called portable power sources for personal computers and portable terminals and vehicle driving power sources. In particular, lithium ion secondary batteries that are lightweight and can afford high energy density are preferably used as high output vehicle driving power sources of electric cars and hybrid cars.
In a typical structure of a nonaqueous electrolyte secondary battery, an electrode body in which a positive electrode and a negative electrode face each other through a separator is housed in a battery case together with a non-aqueous electrolyte. The separator is generally formed of a porous sheet of a resin such as polyolefin and has a function of electrically insulating the positive electrode and negative electrode, a function of holding the non-aqueous electrolyte, and a shut-down function. The shut-down function is a function such that when the inside of the battery is overheated to reach a specified temperature range (typically, a softening temperature of the separator), the separator is softened, and a conduction path of charge carriers is shut down. In addition, the separator is also required to have heat-resistance (endurance) of a specified level. That is, even when the inside of the battery is overheated to the softening temperature or more of the separator and the separator is subject to thermal shrinkage or breakage, it is necessary to prevent the internal short circuit from occurring. As means for responding to such a requirement, a structure in which a porous heat resistance layer (HRL) is formed on a surface of a resinous porous separator is proposed. The porous heat resistance layer is typically mainly formed of particles of an inorganic compound (inorganic filler) and has high heat resistance and insulation properties (nonconductive properties). In WO2008/149895 A, for example, a structure where a polyolefin microporous membrane having the porosity of 35 to 50% by volume is used as a separator, and a porous heat resistance layer is formed on a surface of the separator is disclosed.